The present invention relates to an optical fiber semiconductor device effective as a laser light source, i.e., a combination of an optical fiber and a semiconductor laser. According to the present invention, an optical fiber semiconductor device is produced at a low cost and an output thereof can be increased. Therefore, the device can be used for various purposes, not only for communications.
When a semiconductor laser diode (semiconductor laser) of edge emitting type and an optical fiber are optically coupled, optical means for converting a divergent beam output from the semiconductor laser to a beam which can be easily input to the fiber is required in order to suppress coupling loss. Further, it is important that these elements (the semiconductor laser, the optical means and the optical fiber) be accurately positioned.
A laser beam output from a semiconductor laser has a spread of an angle of about xc2x120xc2x0 in the vertical direction and about xc2x15xc2x0 to 10xc2x0 in the horizontal direction in free-space propagation in the air. Therefore, when the edge of a semiconductor laser and an optical fiber are directly connected (butt joint), coupling loss of about 10 dB is caused. Therefore, to reduce the coupling loss, the beam is converted by optical means such as a lens. Further, in optical communication for the purpose of long-distance and large-capacity communication, a single mode fiber having a core diameter of 10 xcexcm or less is used. Therefore, when this type of optical fiber is coupled to a semiconductor laser, they must be positioned with a high accuracy of 1 xcexcm or less.
An example of the conventional semiconductor laser module for use in optical communication will be described with reference to FIG. 1.
xe2x80x9cFoundation and Application of Optical Coupling System for Optical Devicexe2x80x9d, Kenji Kono, GendaiKogakuSha, page 89 shows a schematic cross sectional view and an external view of a semiconductor laser module. The same publication, page 77 discloses a lens structure of the module of this type.
The semiconductor laser module has an InGaAsP/InP semiconductor LD of an oscillation wavelength of 1.29 xcexcm. The module includes a sphere lens 1 and GRIN (graded-index) rod lenses 2-1 and 2-2. The sphere lens 1 and the rod lens 2-1 are solder-fixed to a metal holder.
A single mode fiber SMF is fixed to a ferrule. The ferrule and the GRIN rod lens 2-2 are fixed to an integrating holder with machining accuracy without optical adjustment. The integrated structure of the single mode fiber and the GRIN rod lens 2-2 is called a virtual fiber, since it can be regarded as a virtual single mode optical fiber having an enlarged spot size.
The elements of the laser module are assembled and adjusted as follows.
(1) The holder of the sphere lens 1 is fixed to the heat sink of the semiconductor laser.
(2) The semiconductor laser LD and the holder of the sphere lens 1 are hermetically sealed together with nitrogen gas within a package in order to maintain the reliability of the semiconductor laser.
(3) An optical output from the single mode optical fiber is monitored by a power meter. To obtain the maximum optical output, the holder of the rod lens 2-1 is adjusted in a direction perpendicular to the optical axis, and the virtual fiber is adjusted in the direction of the optical axis and a direction perpendicular to the optical axis. At this time, the axial dislocation, which may occur when the holder of the sphere lens 1 is fixed to the heat sink of the semiconductor laser, is automatically corrected.
(4) The holder of the rod lens 2-1 is fixed to the semiconductor laser package.
(5) The virtual fiber is position-adjusted in the direction of the optical axis and a direction perpendicular to the optical axis, and the axial dislocation of the rod lens 2-1 is corrected. Thereafter, first, the virtual fiber is fixed to a holder in the direction of the optical axis. Then, the virtual fiber holder is fixed to the holder of the rod lens 2-1. Thus, the elements are fixed in a direction perpendicular to the optical axis.
As can be understood from the above description, in the case where the aforementioned optical lenses are assembled to couple the semiconductor laser and the optical fiber, a metal package of a coaxial structure is employed to reduce the influence of a change in temperature. Moreover, a number of very accurate adjusting steps are required. In the above example, the virtual fiber is adjusted simultaneously in the direction of the optical axis and a direction perpendicular to the optical axis to adjust the rod lenses in the direction of the optical axis. It is very difficult to adjust a number of parameters at the same time. In addition, an optimum value cannot be easily obtained.
In particular, if the single mode fiber has a diameter of 10 xcexcm or smaller, since the positional accuracy must be 1 xcexcm or less, many parameters must be adjusted simultaneously with high accuracy. For this reason, the semiconductor laser module produced in the above method is very expensive, since the adjustment requires a very long period of time, resulting in an increase in manufacturing cost.
Further, to increase an optical power up to the watt class in a semiconductor laser, it is necessary to extend the active layer. Particularly in a GaAs-based semiconductor, when the optical density is increased, optical damage may occur. Therefore, the optical density is limited to about 1 MW/cm2. Since the thickness of the activation layer is determined to a limited value in order to maintain the light emission efficiency, it is necessary to increase the width of the active layer. Specifically, the width of the active layer is extended to 150 xcexcm to 500 xcexcm to realize a laser in the watt class.
However, in general, it is very difficult to converge by a coupled optical system a light beam emitted from a light source having considerably different dimensions in the vertical direction (the thickness of the active layer of about 0.1 xcexcm ) and the horizontal direction (the width of the active layer of about 150 xcexcm to 500 xcexcm ) to a fiber having a diameter of about, for example, 10 xcexcm. In other words, it is practically difficult to suppress the coupling loss, if a high-power semiconductor laser and an optical fiber are coupled by combining the optical lenses in the aforementioned manner.
As described above, when a high-power semiconductor laser and an optical fiber are coupled by means of an imaging optical system including the conventional optical members, a high cost is required, since a precision structure and accurate adjustment are required. In addition, the connection of an optical fiber with a high-power semiconductor laser arises the serious problem of an increase in coupling loss.
For the reasons stated above, the conventional optical fiber output semiconductor device comprising an imaging optical system, in which small lenses are combined, uses a metal housing to ensure the reliability and requires positional adjustment with high accuracy in assembly time. Therefore, although an LD (laser diode) chip itself is inexpensive, the module as a whole is inevitably very expensive. Moreover, because of the increase in coupling loss, it is practically very difficult to convert a divergent optical beam in the watt class, output from a semiconductor laser of edge emitting type, to an optical fiber beam. Therefore, a high-power optical fiber output semiconductor device has not been realized.
It is accordingly an object of the present invention to provide an inexpensive, high-power optical fiber output semiconductor device.
To achieve the above object, an optical fiber output semiconductor device of the present invention comprises: an edge emitting type semiconductor element having an active layer and an output face; a tapered optical waveguide having a structure for accommodating expansion and contraction due to a change in temperature and a first end butt-joined to the output end of the edge emitting type semiconductor element; and an optical fiber connected a second end of the tapered optical waveguide.
The basic structure of the present invention is the combination of an edge emitting type semiconductor chip, a tapered optical waveguide for accommodating expansion and contraction, and an optical fiber.
The optical fiber output semiconductor device of the present invention may have the following features. The tapered optical waveguide may be made of silicone material. A guide groove may be formed on the edge emitting type semiconductor chip. The semiconductor chip and the optical waveguide may be assembled by means of self-aligning and a laser process. If an SLD (super luminescent diode) is used, it may be combined with an external mirror resonator by means of a laser self-aligning process. Alternatively, the semiconductor chip and the optical waveguide may be assembled by means of self-aligning and a resist process.
In the optical fiber output semiconductor device, assuming that a z-axis represents a direction of travel of light, and an x-axis and a y-axis represent two axes on planes perpendicular to the z-axis, the width of the tapered optical waveguide along the x-axis decreases with travel of the light and the width thereof along the y-axis increases with travel of the light. The index of refraction of the core portion of the tapered optical waveguide may decrease as the thickness of the core portion increases. The core portion may comprise a plurality of thin films having different indexes of refraction. A plane wave in the width direction may be converted to a spherical wave. To obtain a graded index, the core portion may include a graded index lens. The graded index lens may comprise a plurality of thin films having different indexes of refraction. An antireflection film may be formed on the output end of the semiconductor chip. The active layer of the edge emitting type semiconductor chip may be tapered in the width direction. The edge emitting type semiconductor element may comprise two emission edges, each connected to a tapered optical waveguide and an optical fiber.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.